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Jacques Gauthier, who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping flight, and the birds that descended from them.

As scientists have discovered more theropods closely related to birds, the previously clear distinction between non-birds and birds has become blurred.

Birds, also known as Aves, are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton.

Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them.

But birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling.

Some birds, especially corvids and parrots, are among the most intelligent animals; several bird species make and use tools, and many social species pass on knowledge across generations, which is considered a form of culture. Birds are social, communicating with visual signals, calls, and bird songs, and participating in such social behaviours as cooperative breeding and hunting, flocking, and mobbing of predators.

Recreational birdwatching is an important part of the ecotourism industry.

Aves and a sister group, the clade Crocodilia, contain the only living representatives of the reptile clade Archosauria.

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